Wideband Code Division Multiple Access (WCDMA) is one of the main technologies for implementation of 3rd generation cellular systems. WCDMA can use Frequency Division Duplex (FDD) as its duplexing technique in connection with its data transmission activities. In these types of systems, WCDMA FDD typically operates with both a receiver and a transmitter that are active at the same time.
An external duplexer filter is typically used in FDD systems. Yet, power levels associated with transmitter leakage at the receiver input can still run undesirably high, e.g. between −30 to −20 dBm. Transmitter leakage that is present at the receiver's front end sets high linearity requirements for the system's mixer In the past, to relax the mixer's linearity requirements, an external SAW filter has been used. Typically, this external SAW filter is placed between the system's low noise amplifier (LNA) and the mixer.
Using an external SAW filter in this capacity, however, has several drawbacks. For example, its insertion loss degrades the noise figure of the receiver. In addition, the SAW filter requires the LNA to have a 50 Ohm output matched, which requires additional matching circuitry. Further, using a SAW filter reduces the level of integration of the receiver, as will be appreciated by the skilled artisan.